According to present-day terminology the term "yarns" covers both simple spinning fibre yarns which are made in a spinning machine by twisting spinning fibres and also twisted yarns which are made by twisting two or more single yarns. Ropes or cables are made by turning or plaiting a plurality of strands which in turn can be turned or plaited from a plurality of yarns. In the special case of metal cables (wire ropes) the basic components are not spinning fibres but wires.
In the production and processing of yarns and ropes various properties must be measured or monitored as far as possible continuously. These include in particular the following properties:
1. the diameter (the thickness) and the fluctuations thereof; PA0 2. the unroundness (deviation from the circular form) and the fluctuations thereof; PA0 3. the frequency and size of projecting individual filaments or wires; PA0 4. the average number of twists or turns per unit length and the fluctuations thereof; PA0 5. the elongation, which can be derived from the change of the number of twists per unit length.
In yarns, these parameters are not only responsible for the quality of the textiles made with these yarns but also for a number of subsequent working processes. For example, the dye absorption depends on the number of twists per unit length. Fluctuations of this parameter cause visible and disturbing fluctuations in colour shades, in particular with plastic yarns.
In the case of ropes monitoring these parameters makes it possible to increase reliability and detect aging processes at an early stage. This is particularly important in conveying apparatus for transporting persons, for example in mining. In the case of many technical yarns and ropes the elongation must be continuously determined. The determination of the frequency and size of projecting filaments or wires is an indication of filament or wire breakages which impair the strength.
Various methods and arrangements are known with which in each case one of the above parameters can be measured or monitored, although usually under restrictive conditions. Thus, capacitive methods are known for monitoring the thickness fluctuations of yarns in which the yarn to be tested lies between the electrodes of a capacitor and thus forms part of the dielectric. These capacitive methods operate contactless and without destruction and are also suitable for monitoring moving yarns. Such methods cannot however measure the twisting, projecting filaments or unroundness. They are moreover sensitive to disturbing influences, such as moisture and density changes, and they fail in the case of metal materials. Furthermore, the measurement of relatively large diameters, for example as occur in ropes, can be made only with difficulty.
So far, the problem which has been solved in the least satisfactory manner is that of determining the number of twists or turns per unit length. For this purpose purely mechanical systems exist in which a yarn specimen is clamped and untwisted until the individual fibres or filaments lie in parallel. Another known apparatus in which with the aid of a punctiform optical measuring head the number of twists per unit length is determined on a yarn specimen is also only a laboratory measuring instrument for evaluating short yarn specimens and not a process data acquisition apparatus for continuous use with running yarns in their production or processing, let alone for monitoring ropes in operation. For monitoring some properties of metal ropes electromagnetic methods also exist but they cannot detect all the parameters of interest and are of course not suitable for non-metallic yarns or ropes.
A further disadvantage of the methods hitherto used for measuring the twisting of running yarns or ropes is that from the running yarn or rope a time signal must be derived and consequently the speed is incorporated into the method. The exact determination of the speed is however not possible in slip-free manner with the usual mechanical means, for example a pulley. For this reason it is also not possible with these known methods to determine the number of twists per unit length and the elongation exactly and independently of the speed. To overcome this disadvantage it is also known to apply markings to the measured object. Thus for example shaft conveying ropes are frequently provided with counting wires which are twisted into the wire cable so that they can be followed from the outside. The attachment of such markings is however restricted to a few specific cases.